The present invention relates, in general, to fossil or other organic fuel boilers and, in particular, to a new and useful method of optimizing the scheduled timing of sootblowing in such boilers.
The combustion of fossil for the production of steam or power generates a residue broadly known as ash. All but a few fuels have solid residues, and in some instances, the quantity is considerable (see Table I).
For continuous operation, removal of ash is essential. In suspension firing the ash particles are carried out of the boiler furnace by the gas stream and form deposits on the tubes in the gas passes (fouling). Under some circumstances, the deposits may lead to corrosion of these surfaces.
Some means must be provided to remove ash from the boiler surface since ash in its various forms may seriously interfere with operation or even cause shutdown. Furnace wall and convection-pass surfaces can be cleaned of ash and slag while in operation by the use of sootblowers using steam or air as a blowing medium. The sootblowing equipment directs air or product steam through retractable nozzles aimed at the areas where deposits accumulate.
The convection pass surfaces in the boiler, sometimes referred to as heat traps, are divided into distinct sections in the boiler (see FIG. 1). Each heat trap normally has its own dedicated set of sootblowing equipment. Usually, only one set of sootblowers is operated at any time, since the sootblowing operation consumes product steam and at the same time reduces the heat transfer rate of the heat trap being cleaned.
Scheduling and sequencing of sootblowing is usually implemented with timers. The timing schedule is developed during initial operation and startup of the boiler. In addition to timers, critical operating parameters, such as gas side differential pressure, will interrupt the timing schedule when emergency plugging or fouling conditions are detected.
TABLE I ______________________________________ COMMERCIAL FUELS FOR STEAM PRODUCTION Fuels Containing Little or Fuels Containing Ash No Ash ______________________________________ All coals Natural gas Fuel oil - "Bunker C" Manufactured gas Refinery Sludge Coke-oven gas (clean) Tank residues Refinery gas Refinery coke Distillates Most tars Wood and wood products Other vegetable product Waste-heat gases (most) Blast-Furnace gas Cement-kiln gas ______________________________________
The sequencing and scheduling of the sootblowing operation can be automated by using controls. See U.S. Pat. No. 4,085,438 to Butler Apr. 18, 1978, for example. The scheduling is usually set by boiler cleaning experts who observe boiler operating conditions and review fuel analyses and previous laboratory tests of fuel fouling. The sootblower schedule control settings may be accurate for the given operating conditions which were observed, but the combustion process is highly variable. There are constant and seasonal changes in load demand and gradual long term changes in burner efficiency and heat exchange surface cleanliness after sootblowing. Fuel properties can also vary for fuels such as bark, refuse, blast furnace gas, residue oils, waste sludge, or blends of coals. As a result, sootblowing scheduling based solely on several days of operating cycles may not result in the most economical, long-term operation of the boiler.
Present practice for sootblowing scheduling is based on the use of timers. The timing schedule is developed during initial operation and start-up. No one timing schedule can be economically optimum, for there are constant and seasonal changes in burner efficiency and heat exchange surface cleanliness after sootblowing.
A boiler diagnostic package which can be used for sootblowing optimization has been proposed by T. C. Heil et al in an article entitled "Boiler Heat Transfer Model for Operator Diagnostic Information" given at the ASME/IEEE Power Gen. Conference in October 1981 at St. Louis, Mo. The method depends upon estimates of gas side temperatures from coupled energy balances, and the implementation requires extensive recursive computations to solve a series of heat trap equations. This method is used to estimate method is used to estimate heat transfer fouling factors. These intermediate results are then used as input to a boiler performance model based on steady state design conditions to estimate cost savings resulting from sootblower initiation. There is no economic optimization, however, and the method does not account for dynamic changes in incremental steam cost (i.e. the cost to produce an additional unit of steam).